PLL circuit having a low-pass passive filter coupled to a varactor diode

Abstract

A low-pass passive filter is inserted in the phase locked loop of a voltage-controlled oscillating circuit having a varactor diode. The filter has a first resistor, a capacitor and a second resistor connected in that order and the blocking characteristics of the filter are changed in accordance with a selected value of the second resistor. A push-pull output circuit in which the upstream transistor is connected directly to the supply voltage line receives the signal from a phase comparator circuit (which is indicative of the result of phase comparison) and outputs it to the low-pass passive filter. The low-pass passive filter controls the terminal voltage of the varactor diode in accordance with the terminal voltage of the capacitor and the value of the second resistor is selected depending on the desired blocking characteristics of the filter.

What is claimed is: 
     
       1. A phase-locked loop circuit comprising: a voltage-controlled oscillating circuit that has a varactor diode and a frequency of oscillation which varies in accordance with a terminal voltage of said varactor diode;   a phase comparator circuit that compares a phase of either an output signal from a reference signal oscillator or an externally supplied input signal with a phase of an output signal from said voltage-controlled oscillating circuit;   a push-pull output circuit that has an upstream transistor connected directly to a supply line and which, in response to a signal from said phase comparator circuit that is indicative of a result of the comparison, allows a current to flow out of said supply line to an output terminal and then sink to ground; and   a low-pass passive filter that has a series circuit provided between said output terminal and said ground, with a first resistor, a capacitor, a second resistor and a third resistor connected in that order, that generates a signal for the terminal voltage of said varactor diode in accordance with a terminal voltage of said capacitor, and wherein blocking characteristics of said filter are changed by selective inclusion of said second and third resistors.   
     
     
       2. A phase-locked loop circuit according to claim 1 which further includes said reference signal oscillator and a switching circuit that is connected to the point of junction between said second resistor and said third resistor, said phase comparator circuit comparing the phase of the output signal from said reference signal oscillator with the phase of an output signal from said voltage-controlled oscillating circuit, and wherein the blocking characteristics of said filter are altered by turning on said switching circuit. 
     
     
       3. A phase-locked loop circuit according to claim 2 wherein said switching circuit is connected between said ground and the point of junction between said second and third resistors. 
     
     
       4. A phase-locked loop circuit according to claim 3 which further includes a programmable divider provided between the input of said phase comparator circuit and the output of said voltage-controlled oscillating circuit, an integrating circuit provided between the output of said low-pass passive filter and said voltage-controlled oscillating circuit, and a controller and wherein the voltage of said integrating circuit is sent to the terminal of said varactor diode whereas said controller sends a dividing signal to said programmable divider and, depending on an applied external frequency selection signal, said controller also sends a control signal that turns on said switching circuit. 
     
     
       5. A phase-locked loop frequency synthesizer comprising: a voltage-controlled oscillating circuit that has a varactor diode and a frequency of oscillation which varies in accordance with a terminal voltage of said varactor diode;   a reference frequency oscillator;   a phase comparator circuit that compares a phase of an output signal from said reference signal oscillator with a phase of an output signal from said voltage-controlled oscillating circuit;   a push-pull output circuit that has an upstream transistor connected directly to a supply line and which, in response to a signal from said phase comparator circuit that is indicative of a result of the comparison, allows a current to flow out of said supply line to sink to ground; and   a low-pass passive filter that has a series circuit provided between said output terminal and said ground, with a first resistor, a capacitor, a second resistor and a third resistor connected in that order, that generates a signal for the terminal voltage of said varactor diode in accordance with a terminal voltage of said capacitor, and wherein blocking characteristics of said filter are changed by selective inclusion of said second and third resistors, and wherein the selective inclusion is determined by a control signal.   
     
     
       6. A phase-locked loop frequency synthesizer according to claim 5 which further includes a programmable divider provided between the input of said phase comparator circuit and the output of said voltage-controlled oscillating circuit, an integrating circuit provided between the output of said low-pass passive filter and said voltage-controlled oscillating circuit, a microprocessor and a switching circuit and wherein the voltage of said integrating circuit is sent to the terminal of said varactor diode whereas said microprocessor sends a dividing signal to said programmable divider and, depending on an applied external frequency selection signal, said microprocessor also sends said control signal. 
     
     
       7. A radio receiver comprising: a voltage-controlled oscillating circuit that has a varactor diode and a frequency of oscillation which varies in accordance with a terminal voltage of said varactor diode;   a reference frequency oscillator;   a phase comparator circuit that compares a phase of an output signal from said reference signal oscillator with a phase of an output signal from said voltage-controlled oscillating circuit;   a push-pull output circuit that has an upstream transistor connected directly to a supply line and which, in response to a signal from said phase comparator circuit that is indicative of a result of the comparison, allows a current to flow out of said supply line to an output terminal and then sink to ground;   a low-pass passive filter that has a series circuit provided between said output terminal and said ground, with a first resistor, a capacitor, a second resistor and a third resistor connected in that order, that generates a signal for the terminal voltage of said varactor diode in accordance with a terminal voltage of said capacitor, and wherein blocking characteristics of said filter are changed by selective inclusion of said second and third resistors; and   a mixing circuit for receiving said output signal from said voltage-controlled oscillating circuit and a high-frequency amplified reception signal, and wherein the selective inclusion is determined by a control signal.   
     
     
       8. A radio receiver according to claim 7 which further includes a programmable divider provided between the input of said phase comparator circuit and the output of said voltage-controlled oscillating circuit, an integrating circuit provided between the output of said low-pass passive filter and said voltage-controlled oscillating circuit, a microprocessor and a switching circuit and wherein the voltage of said integrating circuit is sent to the terminal of said varactor diode whereas said microprocessor sends a dividing signal to said programmable divider and, depending on an applied external frequency selection signal, said microprocessor also sends said control signal.